1. Field of the Invention
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode having well and/or barrier layers with a superlattice structure.
2. Description of the Related Art
In general, since Group-III-element nitrides, such as GaN, AlN and InGaN, have an excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in blue and ultraviolet regions. Particularly, an InGaN compound semiconductor has been considerably noticed due to its narrow band gap. LEDs using such a GaN-based compound semiconductor are used in various applications such as large-sized full-color flat panel displays, backlight sources, traffic lights, indoor illumination, high-density light sources, high-resolution output systems and optical communications. Further, LEDs emitting near ultraviolet are used in forgery identification, resin cure, ultraviolet therapy and the like, and can implement visible light with various colors through combination with phosphors.
FIG. 1 is a sectional view illustrating a conventional LED.
Referring to FIG. 1, the LED comprises an N-type semiconductor layer 17, a P-type semiconductor layer 21 and an active region 19 interposed between the N-type and P-type semiconductor layers 17 and 21. The N-type and P-type semiconductor layers include Group-III-element nitride semiconductor layers, i.e., (Al, In, Ga)N-based compound semiconductor layers. Meanwhile, the active region 19 is formed to have a single quantum well structure having a single well layer, or a multiple quantum well structure having a plurality of well layers, as shown in this figure. The active region with a multiple quantum well structure is formed by alternately laminating InGaN well layers 19a and GaN or AlGaN barrier layers 19b. The well layer 19a includes a semiconductor layer with a smaller band gap than the N-type and P-type semiconductor layers 17 and 21 and the barrier layer 19b, thereby providing quantum wells in which electrons and holes are recombined with each other.
Such a Group-III-element nitride semiconductor layer is grown on a different-type substrate 11, such as sapphire with a hexagonal system structure or SiC, using a method, such as organic chemical vapor deposition (MOCVD). However, if a Group-III-element nitride semiconductor layer is grown on the different-type substrate 11, a crack or warpage occurs in the semiconductor layer and dislocation is produced due to the difference of lattice constants and thermal expansion coefficients between the semiconductor layer and the substrate.
In order to prevent these problems, a buffer layer is formed on the substrate 11. The buffer layer generally includes a low-temperature buffer layer 13 and a high-temperature buffer layer 15. The low-temperature buffer layer 13 is generally formed of AlxGa1-xN (0≦x≦1) at a temperature of 400 to 800° C. using a method, such as MOCVD. The high-temperature buffer layer 15 is then formed on the low-temperature buffer layer 13. The high-temperature buffer layer 15 is formed of a GaN layer at a temperature of 900 to 1200° C. Accordingly, crystal defects in the N-type GaN layer 17, the active region 19 and the P-type GaN layer 21 can be considerably removed.
However, although the buffer layers 13 and 15 are employed, crystal defect density in the active region 19 is still high. Particularly, in order to enhance a recombination efficiency of electrons and holes, the active region 19 includes a semiconductor layer with a smaller band gap than the N-type and P-type GaN layers 17 and 21. In addition, the well layer 19a includes a semiconductor layer with a smaller band gap than the barrier layer 19b, and generally contains a larger amount of In. Since In is relatively larger than Ga and Al, the lattice constant of a well layer is relatively greater than that of a barrier layer. Therefore, lattice mismatch occurs between the well layer 19a and the barrier layer 19b, and between the well layer 19a and the N-type semiconductor layer 17. Such lattice mismatch between the layers causes pin holes, surface roughness and the like, and degrades crystal quality of the well layer, thereby restricting the light efficiency.